A rebreather is a device that retains and reuses some or all of a user's expired breath. Even with physical exertion, a person uses only a fraction of the oxygen that is inhaled. A rebreather recirculates unused oxygen in the system and replenishes the oxygen consumed by the wearer. This allows a very small tank of oxygen to last much longer than is possible using traditional self contained breathing apparatus (SCBA) gear. Three main components of typical rebreather systems include a gas supply/oxygen control, counterlung, and carbon dioxide removal system.
The carbon dioxide removal system maintains carbon dioxide CO2 (CO2) pressures at a safe level. Maintaining CO2 at safe pressure levels is relatively easy to do, and is accomplished by passing exhaled gases through a canister filled with a chemical adsorbent, such as soda lime and anhydrous lithium hydroxide (LiOH). Several manufacturers make these adsorbents and use their own special mixes. For example, SODASORB®, manufactured by W. R. Grace & Co., is composed of a mixture of sodium hydroxide, calcium hydroxide, and potassium hydroxide. Granular LiOH absorbents are typically used in submarine, space, and emergency CO2 removal equipment, where adsorbent volume and weight is maintained at minimal levels.
Adsorbents are typically in the form of small granules that are generally sized between 0.04 to 0.25 inches (1.0 to 6.5 mm) in diameter. The granules may be placed in a canister through which exhaled gases are passed. Smaller granules allow more surface area per unit weight, but because a person must “breathe” through this canister without too much resistance, larger adsorbent particles are employed so as to allow gas flow around these granules and through the canister with a relatively low pressure drop. Thus, one of the limitations of current adsorbent canisters is the relatively large adsorbent particle size necessary to obtain low pressure drops and, in turn, ease of breathing. Additionally, powered or forced air systems typically have low system pressure drop requirements and as a result low adsorbent pressure drops are required. Airflow, gas flow, or the flow of air or gas is defined as the motion of air around an object, not including diffusion. Airflow may be forced air (e.g., via a fan or human respiration) or passive (e.g., thermal convection).
High CO2 removal rates are needed to reduce the size and weight of SCBAs while maintaining acceptable CO2 levels. Once acceptable CO2 levels are exceeded, the adsorbent cartridge is replaced. A 0.5 percent CO2 outlet concentration is typically used to define the replacement point. The CO2 removal capacity can be defined as the total amount of CO2 removed, length of time used (i.e., duration), or both total CO2 removed and duration. Decreasing granule size can increase removal rates (Davis 1978). However, decreasing granule size results in increased pressure drop. System design usually involves trade-off between CO2 removal rate, system pressure drop, CO2 removal capacity, and system size.
LiOH operates to remove CO2 in the following manner. The reaction of CO2 with anhydrous LiOH requires the presence of water (Wang) that, when used in a self-contained breathing apparatus, is provided by exhaled gas in the form of water vapor. The LiOH combines with the water vapor to form lithium hydroxide monohydrate (2LiOH*H2O) and is highly exothermic (i.e., generates heat. The reaction of LiOH and water is described in Eqn. (1):2LiOH(solid)+2H2O(gas)→2LiOH*H2O(solid), which produces −29.04 kcal/2 moles LiOH of energy.   Eqn. (1)
The exothermic monohydrate reaction of Eqn. (1) provides heat needed for an endothermic reaction (i.e., absorbs heat) with CO2, and is described in Eqn. (2):2LiOH*H2O(solid)+CO2(gas)→Li2CO3(solid)+3H2O(gas), which absorbs +7.65 kcal/mole CO2 of energy.   Eqn. (2)
The overall reaction is given in Eqn. (3) and is also exothermic:2LiOH(solid)+CO2(gas)→Li2CO3(solid)+H2O(gas), which produces −21.39 kcal/mole CO2 of energy.   Eqn. (3)
Because the LiOH is anhydrous (i.e., substantially without water), the initial source of the water that is needed for the reaction with CO2 is water vapor in an incoming gas stream. The source of the water vapor in the incoming gas stream includes ambient water vapor (i.e., ambient humidity) and/or vapor from human respiration. Davis showed that the CO2 removal efficiency of anhydrous LiOH granules falls rapidly under dry conditions.
Although water is needed for the reaction between LiOH and CO2, too much water vapor has been shown to be detrimental to the reaction. The CO2 removal efficiency of anhydrous LiOH granules improves as humidity is increased up to an optimum humidity level (Davis 1978). Above this level, the CO2 removal efficiency decreases. The presence of an optimum humidity level indicates that there is an optimum operating water content for LiOH granules.
While anhydrous LiOH requires hydration to have a CO2 reaction, the use of pre-hydrated porous LiOH granules (i.e., LiOH granules having an initial water content above an anhydrous level) has also been studied by Davis (1980). In this study, it was concluded that there is not a significant difference in the reactivity between wet and anhydrous LiOH granules. Without a CO2 removal benefit, pre-hydration of granules only increases the weight of the adsorbent, which is not desirable. Wang demonstrated that the CO2 adsorption capacity was generally lower by approximately 10 percent for partially hydrated (e.g., 5.45 percent water-by-weight) LiOH granules. These studies indicate that the pre-hydration of LiOH granules provides no benefit and may be even detrimental for CO2 removal and adsorbent weight.
In summary, although water is important for the CO2 removal reaction, Wang, Davis and others have shown that only water in vapor form is beneficial and too much water, even in vapor form, is detrimental to the reaction of LiOH and CO2. Pre-hydration of the LiOH has been shown to be ineffective at best and detrimental at worst.
As indicated in Eqn. 1, the addition of water to anhydrous LiOH results in an exothermic reaction, which generates heat. Additionally, the overall reaction in the reaction as described in Eqn. (3) shows heat generation. Because the heat given off in the reaction described in Eqn. (1) is 3.8 times as high as the heat absorbed in the reaction described in equation (2), the generation of heat is dominated by the reaction of LiOH with water. The main driving force for the rate of heat generation is the rate at which LiOH reacts with water. In general, the use of LiOH for CO2 removal operates effectively for removing large amounts of CO2, but drops off relatively quickly after an initial reaction (see FIG. 19, between points A and B).
The main driving force for the total amount of heat generated is the total amount of LiOH reacted with water. In closed systems, such as submarine CO2 removal systems, the heat generated can result in adsorbent exhaust temperatures as high as 160 degrees Fahrenheit. Adsorbent surface temperatures are even higher. This level of heat generation results in excessive breathing temperatures for closed systems, such as a rescue hood used by firefighters, is detrimental in most applications. One technique for reducing adsorbent system exhaust temperature includes utilizing molecular sieves downstream of CO2 removal systems, but are problematic due to added weight, size, cost, and breathing resistance.